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Things that Go Bump in the Night: Is Your Furnace Making Noises?


It’s that time of year again when you see skeletons and witches inhabiting every lawn and peering out of every window. So when you have a furnace making noises with all sorts of unsettling sounds, you can’t help but wonder if a poltergeist got in there.

Before you call in the ghost busters, here are some of the common problems that may be making your heating system go bump in the night (and all day long). Luckily, most of them are not that scary.

The Whistle

That whistling sound that creeps out everyone in the building is not a banshee, but an air flow problem in your ventilation system. Some possible causes:

  • Clogged filter. When your furnace’s air filter gets clogged with dust and debris, after a while it’s preventing adequate air from flowing through the system. This can cause pressure problems that result in that unnerving noise. This one is simple to fix: replace the air filter and your problem should disappear in a hurry.
  • Holes in the ducts. Over time, holes and cracks can appear in your ventilation ducts. When the air moving through the ducts leaks out of the cracks, it can make some eerie sounds. Call in your friendly neighborhood HVAC service guy to repair the duct, and restore peace and quiet to your building.
  • Inadequately sized ductwork. Did you get a new furnace without updating your ventilation system? If so, your ducts may be too small to handle the volume of air being forced through them. You’ll need a certified HVAC expert to evaluate your system’s air flow requirements and install the correct size ducts.

The Rattle

Is all that shaking and rattling making people wonder if there’s an impending earthquake or a zombie apocalypse? The first step to diagnosing the cause is determining where the sound is coming from. Is it the furnace or the ventilation ducts? In most cases, you will need to call in the experts to banish this noise from your building.

  • Ducts are loose. Your ventilation ducts are supposed to be securely attached to the wall, floor or ceiling but over time vibration can cause fastenings to come loose. The air pressure can cause loose ducts to make a racket. If you can find the spot where the duct has come loose, you may be able to fix this with a screwdriver.
  • Something in ducts. Have you got kids in the building, or anyone with a mischievous sense of humor? It’s not unheard of for unusual objects to end up in your ducts, causing that horrendous sound.
  • Your furnace needs to be stabilized. Is the whole furnace shaking? Buildings settle over time, and it may no longer be adequately stable on its surface. Don’t try to fix this yourself! Don’t forget you are dealing with a combustion appliance, and probably a gas line. Get the experts to take care of this.
  • Bearings are wearing out. If the noise is coming from the furnace motor, the motor bearings are probably wearing out and may need to be replaced.

The Squeal

If it sounds like a tortured spirit is living in your furnace, it may have developed maintenance issues, especially if it has not been serviced in a while. Here’s the probable cause:

  • The blower belt is failing. The belt that drives the blower motor may have developed a problem. It may have slipped, come loose or it may be worn and needs to be replaced. During an HVAC preventative maintenance inspection, this would have been fixed long before it developed that annoying squeal
  • Motor shaft bearings need oiling. Again, this is a problem that is easily preventable with regular maintenance. Your furnace’s moving parts need to be oiled every so often so they continue to work efficiently and quietly.

The Bang

This one could actually be a hair-raising situation. If you hear a fairly loud bang every time your furnace turns on, call an HVAC expert right away. The problem could turn out to be minor, but it could also be a problem with your gas ignition, which is nothing to fool around with.

Here are some possible causes:

  • Gas ignition problem. That “bang” could actually be a small explosion that happens every time your furnace ignites. It’s probably due to dirty burners that delay the ignition and cause gas to build up before it finally ignites. It could also be a faulty electronic ignition. You need to get it checked out right away! These little explosions can actually crack your heat exchanger, which could allow carbon monoxide to escape into your building. It will also mean you need a new furnace.
  • Ducts expanding. If your duct work has a weak spot, the pressure changes that occur when the furnace turns on can force it to “pop” or expand in that weak area. Call in an HVAC service expert to repair any weak spots, and that banging sound will be history.

Are you starting to see a pattern here? Many of those terrifying (or at least downright annoying) sounds coming from your furnace are caused by a lack of maintenance. Even the worst-case scenario here (the gas ignition problem) would be prevented if the furnace was regularly cleaned and inspected.

So now you know; there’s no need to call in the ghost busters, but you do need to call in a qualified HVAC service company to get rid of those spooks in your system, and keep it running efficiently and reliably.

Peace and quiet is only one of the many costs of failing to regularly maintain your HVAC equipment. Poor maintenance also costs your business in many ways that you may not realize. Want to learn more? Grab a copy of our helpful guide to Calculating the Hidden Costs of Poor HVAC Maintenance.
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